Description:
------------
Per Twitter exchange with @pierrejoye

The exif_read_file function checks whether the file is accessed over stdio. If 
it is accessed over stdio, VCWD_STAT() is used to get the size of the file 
efficiently. If it is accessed by any other means, this pattern is used:


                        php_stream_seek(ImageInfo->infile, 0, SEEK_END);
                        ImageInfo->FileSize = php_stream_tell(ImageInfo-
>infile);
                        php_stream_seek(ImageInfo->infile, 0, SEEK_SET);

Moving the seek pointer to the end can trigger slow, bandwidth-hungry network 
operations for remote stream wrappers, such as S3 wrappers. 

Please test whether the stream implements the stat operation and, if so, use it 
to avoid this performance hit.

I was able to work around the problem by teaching my stream wrapper to 
specifically tolerate the "seek to the end / ftell / seek to the start" pattern 
and return cached data.